@media screen and (min-width: 750px) {
    html {
        font-size: 75px !important;
    }
}
body {
    min-width: 320px;
    width: 10rem;
    margin: 0 auto;
    line-height: 1.5;
    font-family: Arial, Helvetica, sans-serif, STHeiTi;
    background-color: blue;
}

div {
    width: 1rem;
    height: 2rem;
    background-color: brown;
}